The chemicals also had industrial uses including in substances like firefighting foams.

Officially they are named are per- and polyfluoroalkyl substances (PFASs) and were previously referred to as perfluorinated chemicals (PFCs).

The PFAS group includes several sub categories of chemicals but the focus in Australia is on perfluorooctane sulfonate (PFOS), perfluoro-octanoic acid (PFOA) and perfluorohexane sulfonate (PFHxS).

How did they end up in NT water supplies?

The chemicals PFOS and PFOA were previously used extensively in firefighting foams by both civilian and Defence Force firefighters around Australia.

These chemicals subsequently leaked from the foams into soils, surface and ground waters.

Prior to 2004, Defence personnel used the toxic foams at the RAAF Base Darwin, the RAAF Base Tindal and Robertson Barracks.

In the Northern Territory the firefighting foams were also used at civilian airports including Darwin and Alice Springs, at firefighting training facilities and some industrial sites.

Where have they been found in the NT?

Power and Water Corporation tested drinking water for PFAS chemicals on a range of urban water supplies including Katherine, Adelaide River, Batchelor, Alice Springs and Darwin regions in October this year.

Some of the early tests results are back and were given to the Health Department.

The Katherine town drinking water is made up of around 80 to 90 per cent water from the Katherine River, which is then blended with the bore water.

Early results showed Alice Springs and Darwin water supplies were clear.

The chemicals were also found in shellfish from Darwin's Rapid and Ludmilla Creeks.

The Health Department released a report by Charles Darwin University that confirmed the chemicals were in aquatic life and sediment in the creeks, which are both close to the RAAF Base Darwin and Darwin Airport.

PFAS chemicals were also found on and around RAAF Bases in Tindal and Darwin and at Robertson Barracks, on the outskirts of Darwin.

Why are people worried about PFAS chemicals?

There is global concern about the chemicals because they do not degrade in the natural environment.

The chemicals are known as an "emerging contaminant" and their specific impact on the environment and humans is still being investigated.

The primary exposure pathway for humans is from drinking water.

Other exposure routes can include eating food stuffs produced from impacted land and water systems like fish, poultry, meat and vegetables.

Have NT authorities issued any health advice?

Yes. In Katherine, the Health Department advised residents who rely solely on drinking water from a private bore, particularly pregnant women, to drink bottled water as a precaution.

The Department also said that because the contamination has mostly like come from RAAF Base Tindal, those residents should contact the Defence Department National Hotline 1800 365 414 to ask if alternative water can be provided.

The Department also advised people not to eat fish from Rapid or Ludmilla Creeks until more tests are done.

Where does the NT health advice come from?

The Northern Territory Health Department based its advice on interim guidelines from a Federal Government body, called the Environmental Health Standing Committee (enHealth).

This advice is considered to be interim because the Federal Health Department commissioned Food Standards Australia New Zealand to publish final reference values for PFOS, PFHxS and PFOA. The FSANZ values that will replace the interim ones.

The interim advice was based on 2008 European Food Safety Authority values.

The drinking water values for PFOS/PFHxS is set at 500,000 parts per trillion or 0.5 micrograms per litre.

What are the international standards?

In November 2016 the United States Environment Protection Authority published a Drinking Water Health Advisory.

The US EPA established health advisory levels for PFOA and PFOS at 70 parts per trillion (ppt) or 0.07 micrograms per litre.

The conversion from ppt to micrograms per litre has been verified by authorities.

Online conversion calculators can be unreliable when converting the ppt figure into micrograms per litre.

In 2009, PFOS (perfluorooctanesulfonic acid) was one of the eleven chemicals which were added to the annexes of the Stockholm Convention on Persistent Organic Pollutants (Stockholm Convention) to which Australia is a signatory.

Where to from here?

Next year, the Defence Department will undertake full investigations into contamination at 12 sites around Australia, including the three in the Northern Territory.

The NT Government established a PFAS interagency working group (PFASIWG) in April 2016 to coordinate an approach investigations and responses to potential environmental and health issues related to PFAS.

Its members include representatives from NT government agencies including the NT Department of Health (DoH), NT Department of Primary Industry and Resources, the Northern Territory Environment Protection Authority (NT EPA), the Commonwealth Departments of Defence and Infrastructure, Darwin International Airport and Air Services Australia.

The Health Department said it will update or issue health advice if needed.